The present invention relates to surface-treated particles, a method for producing surface-treated particles, and use thereof, in particular the surface treatment of titanium dioxide, barium sulphate, zinc sulphide or lithopone particles, and mixtures of these particles with specific alkoxylated siloxanes and phthalate-free plasticisers for improving dispersion in plastics.
Plastics which are formulated so as to produce corresponding polymer compositions are divided into the main groups of thermoplastics and thermosets.
Here, polymers that have a flow transition range above the working temperature are referred to as thermoplastics. Thermoplastics are linear or branched polymers which in principle are flowable above the glass transition temperature (Tg) in the case of amorphous thermoplastics and above the melting point (Tm) in the case of (partly) crystalline thermoplastics. In the softened state they can be processed by pressing, extrusion, injection moulding, or other shaping methods to form moulded articles. The chain movability is so large here that the polymer molecules slide easily over one another and the material reaches the molten state (flow range, polymer melt). Thermoplastics additionally also include thermoplastically workable plastics having pronounced entropy-elastic properties, or what are known as thermoplastic elastomers. Thermoplastics also include all plastics consisting of linear or thermolabile cross-linked polymer molecules, for example polyolefins, vinyl polymers, polyesters, polyacetals, polycarbonates, sometimes also polyurethanes and ionomers, but also TPEs (thermoplastic elastomers (RÖMMP ONLINE, version 3.7, Carlowitz and Wierer, Kunstoffe (Merkblätter) (Plastics (Datasheets)), chapter 1, Thermoplaste (Thermoplastics), Berlin: Springer publishers (1987), Domininghaus, page 95 ff).
Thermosets are plastics which are produced by irreversible and close crosslinking from oligomers (technical term: prepolymers), and more rarely from monomers or polymers, via covalent bonds. The term “thermoset” is used here both for the raw materials prior to the cross-linking (see reaction resins) and as a collective term for the cured resins, which are usually completely amorphous. Thermosets are energy-elastic at low temperatures, and even at high temperatures they cannot flow in a viscous manner, but instead behave elastically with very limited deformability. Thermosets include, inter alia, the technically important substance groups constituted by diallylphthalate resins (DAPs), epoxy resins (EPs), urea formaldehyde resins (UFs), melamine formaldehyde resins (MFs), melamine phenol formaldehyde resins (MPFs), phenol formaldehyde resins, and unsaturated polyester resins (UPES) (RÖMMP ONLINE, version 3.7, Becker, G. W.; Braun, D.; Woebecken, W., Kunstoff-Handbuch (Plastics Handbook), volume 10: Duroplaste (Thermosets), 2nd edition; Hanser: Munich, (1988); Elias (6.) 1, 7, 476 ff).
The properties of plastics of this type can be improved by additives. By way of example, plasticisers can be added as additives in order to make the plastics softer, more flexible, more pliable and more elastic for use or for further processing. Plasticisers can be esters of low volatility, fatty oils, or soft resins. Pigments can also be added to the plastics, which pigments are constructed from organic or inorganic constituents. For improved compatibility in particular of the inorganic particles, these can be surface-modified.
Surface-modified particles of this type can consist for example of an inorganic oxide, which is provided with a further inorganic coating as appropriate. Silicon dioxide and aluminium oxide can be used as an inorganic coating, for example of a titanium dioxide pigment. In addition, the inorganically coated pigment can comprise an organic coating in order to influence the hydrophobicity. Organic coating means known from the prior art are, inter alia, PDMS (polydimethylsiloxane) or polyalcohols, such as TMP, or substituted siloxanes, such as alkyl ester polydimethylsiloxane, or reactive coupling agents, such as silanes, zirconates or titanates, or organic compounds containing phosphorus, or stearic acids, Ca stearate, Zn stearate, and Mg stearate. Gilbert thus describes (Gilbert, Varshney, van Soom und Schiller, “Plate-out in PVC Extrusion—I. Analysis of-plate-out”, Journal of Vinyl and Additive Technology, (14) 1, 2008, 3-9) the use of Ca stearate or other soaps or the corresponding carboxylic acids, such as stearic acid, in order to realise a surface treatment of titanium dioxide or barium sulphate or mixtures thereof. These methods may result in undesirable side-effects, since products of this type often either cannot be easily used in large-scale pigment production processes or also lead to what is known as plate-out in the production of highly filled masterbatches, i.e. concentrates of the pigments in a thermoplastic matrix. This is understood to mean the build-up of the thermoplastic pigment mixture on the screw or the interior of the housing of the extruder screw. This results in extensive cleaning, particularly with colour changes. In addition, layers deposited in this way can detach at any time and lead to the formation of specks in the masterbatch and later also in the end application, i.e. for example can lead to specks in thin-layer films.
The use of silicone oils of different chain length/viscosity which are sprayed on either as the pigment is ground or which can be fed as emulsion into a pigment slurry is also known from GB 1 288 581. Pigments with silicone oil surface treatment have good hydrophobicity and, on account of their low surface energy, are generally known for the fact that they can be easily introduced into a low-energy plastics environment. Pigments treated in this way have various disadvantages:                Silicone oil-treated particles have a high dust value. Although, for example, titanium dioxide with a relatively high density of 3.9 g/cm3 should not necessarily tend toward this, the high dust value after silicone oil treatment is known from U.S. Pat. No. 3,649,321. The dust value is very problematic with regard to the further processing and may even be relevant to safety.        A low bulk density of the silicone oil-treated pigments leads to problems with regard to the packaging of the pigment in sacks, big bags, or silos. A high amount of air requires filling with a lower mass per packaging unit. If the sack cannot be stacked flat on a pallet, the strapping effort or the effort involved in film-wrapping the pallets increases. Automatic packaging with air conveyance, which is considered to be a high-performance standard, encounters this problem especially.        Silicone oils as surface treatment can lead to further problems in the production of commodities, for example the unsuitability of films for overpainting, printing and/or welding, and in particular a delamination of layers and functional loss can be observed in multi-layer films (Plastics Additives: Advanced Industrial Analysis, 2006, Jan C. J. Bart, pages 419-420; IOC Press, Netherlands, ISBN 1-58603-533-9; Trouble Shouting Guide Brochure—Siegwerk, March 2013).        
It is known from DE 41 40 793, and EP 0 546 407, EP 0 546 406 that silicone oils or silane-based structures such as methacryloxypropyltrimethoxysilane are suitable for coating oxidic surfaces as well in order to thus enable higher compatibility with the surrounding organic medium via a hydrophobic or functionalisation of the surface and a reduction of the viscosity, and therefore a greater efficiency of the used pigments. Silanes are sensitive to hydrolysis and generally require an increased temperature of approximately 80° C. in order to activate the silyl or functionality at the pigment or filler surface and form a polymer structure on the surface. Many processes in pigment production are aqueous slurry processes, or the pigments such as titanium dioxide produced by precipitation reactions in water, such that there is a need to be able to introduce surface treatment agents in aqueous form, for example emulsified, into the process in simple form. On the other hand, in processes in which the surface of dry pigments or fillers is treated, there is a need for there to be no automatic possibility in these processes to increase the temperature significantly above the ambient temperature, or for there to be no resultant cost increase, although this is a process disadvantage for the binding of silanes because temperatures above 80° C. are necessary for this purpose for splitting-off the alcohol. There was thus a need to provide a surface treatment for dry grinding or mixing processes which, as appropriate, also guarantees surface modification without a rise in temperature. Furthermore, it would be advantageous to use a method in which no alcohol is split off in order to avoid costly explosion protection for the grinding and/or drying procedure.
The dispersion of hydrophilic inorganic pigments, such as titanium dioxide, barium sulphate, zinc sulphide and/or lithopone, which comprise a multiplicity of hydroxyl groups (HO—) and/or thiol groups (HS—) at the surface poses a problem. Either a high outlay is to be expected on account of long dispersion times, or there is an inadequate dispersion with unacceptable defect characteristics of the end product. Defect characteristics of this type can be: the formation of specks in plastics films which are not only aesthetic in nature, but can lead to the formation of holes and therefore leaks in the packaging, specks in the injected plastics part can lead to mechanical defects, i.e. result in a much lower tensile strength or impact strength and therefore lead to a high rejection rate in the injection-moulding production process or even to failure of safety-relevant components made of plastic in the automotive or electronics industry.
As already mentioned before, besides pigments, further additives such as plasticisers can also be added to plastics. Plasticisers are substances which are added to thermoplastic polymers in order to make these softer, more flexible and/or more elastic. By way of example, soft PVC in contrast to hard PVC contains greater quantities of plasticisers and can be easily processed by calendering or extrusion. By varying the plasticiser proportion, the flexibility of the plastic, for example of a PVC film, can be purposefully adjusted. Films of this type are permeable to oxygen and carbon dioxide and have been used for the packaging of food. However, phthalate plasticisers used for this purpose have been found to have a partly teratogenic and fertility-destroying effect. In order to be able to use plasticisers in packagings that have contact with food, plasticisers with reduced migration tendency are necessary. Such plasticisers can then be used for food packagings as well as for medical equipment and children's toys.
Indispensable requirements of pigments for use in polymers are good flowability and low dusting, good dispersibility and fineness (for example subsequent use in very thin films), quick wettability (to increase throughput in paste production), high possible degrees of filling (to reduce the storage volume of the stock pastes) and absolute flocculation and settling stability over the storage period. In addition, approval for potential indirect contact with food and/or medical compatibility is of course required. Surface-treated particles which are to be used in plastics packaging suitable for food are subject to European standard EC 10/2011 and/or the standards of the American FDA.
A wide variety of requirements are placed on pigments. On the one hand, they must be optimised in respect of the end use for processing in plastics, and on the other hand the incorporation process in a plasticiser (for example with a dissolver) and subsequent storage thereof as a paste or liquid as masterbatch requires a pigment which must be optimised for paints and dyes. Both requirements can be taken into account within the scope of an optimised organic coating.
However, particles which meet both the above-mentioned requirements and also additionally avoid the previously mentioned disadvantages are known in the prior art.
The object of the invention is therefore to provide particles for plastics products which meet the above-mentioned requirements, particularly in respect of the approval for food packaging, and in addition eliminate the disadvantages known from the prior art.